Gecko SDK, also known as the XULRunner SDK, is a set of XPIDL files, headers and tools to develop XPCOM components which can then in turn e.g. be accessed from XUL using JavaScript.

Note that in order to develop such components, you do not need the full sources of e.g. Firefox, since you do not access parts of the front end from within a component. Furthermore, since a component contains basic functionality, it is intended to run with every product of the Mozilla platform. Therefore it makes no sense to use a particular product to build general functionality. That's the reason why the Gecko SDK was devised.

Do not confuse the Gecko/XULRunner SDK with XULRunner itself. The Gecko SDK is a collection of header files and tools used to develop general XPCOM components which add functionality to the existing platform, whereas XULRunner is a framework which can be used to run standalone or embedded applications based on the Mozilla framework.

The Gecko SDK contains all of the necessary tools and headers for making scriptable NPAPI plugins including the xpidl compiler/linker and the latest npapi.h.